The present invention generally relates to a method for preventing retinal ganglion cell death, associated with glaucoma, and is more specifically directed to administering to retinal ganglion cells of a mammal, a compound which blocks the reverse mode of the sodium/calcium exchanger.
Glaucoma, a disease of unknown etiology, results in the degeneration of the optic nerve and the subsequent loss of the visual field. In the case of primary open angle glaucoma (PAOG) where intraocular pressure is elevated present therapies rely on lowering pressure in the hope that this may retard the progressive loss of the visual field.
Another approach to prevent the loss of nerve fibers associated with glaucoma is to target the retinal ganglion cell or optic nerve directly, see U.S. Pat. No. 5,922,746. It has been shown that a novel class of voltage-gated sodium Na+ channels and the sodium/calcium exchanger (Na+/Ca2+) residing within the optic nerve are responsible for the damage to rat optic nerve following anoxia or hypoxia. The underlying hypothesis is that elevation of intracellular calcium beyond physiological levels kills retinal ganglion cells and their axons. It is believed that during hypoxia or anoxia when intracellular ATP concentrations decrease the following series of intracellular events occur within the optic nerve tract. As a result of the decreased functioning of the Na/K pump, which is fueled by intracellular ATP, intracellular levels of potassium (K+) and Na+ decrease and increase respectively. Subsequent loss of cell K+ results in an increase in extracellular K+ that subsequently depolarizes the optic nerve axons. Depolarization activates voltage-gated Na+ channels including a unique subset of Na+ channels that do not inactivate during prolonged depolarization. Na+ channels that do not inactivate are known as non-inactivating or persistent Na+ channels. Persistent Na+ channels provide a sustained influx of Na+ into anoxic/depolarized optic nerve tract (Stys et al, 1993, Waxman et al, 1992). Sustained Na+ influx and reduced Na+/K+ pump activity results in an elevated intracellular Na+ concentration, a condition termed xe2x80x9cNa+ overloadxe2x80x9d. Na+ overload and membrane depolarization results in the reverse operation of the electrogenic Na+/Ca2+ exchanger, which normally operates to extrude intracellular Ca2+. Reverse operation of the Na+/Ca2+ exchanger leads to a large increase in intracellular Ca2+ (Ca2+ can increase from nanmolar to micromolar levels). Large increases in cell Ca2+ are believed to be responsible for neuronal cell death triggered by a variety of insults. As such prevention of Ca2+ overload has been demonstrated to be neuroprotective in CNS neurons including rat optic nerve (Stys et al, 1993).
The present invention is directed to preventing optic nerve malfunction during following ischemia/hypoxia by inhibiting the Na+/Ca2+ exchanger. As mentioned above, under hypoxic/ischemic conditions the exchanger is running in the reverse mode, thus loading the cell with Ca2+ (see FIG. 2).
On the other hand, Na+/Ca2+ exchangers of retinal ganglion cells (RGCs), their associated axons (that make up the optic nerve), and other retinal cells, that are not ischemic, depolarized, or overloaded with Na+, will possess Na+/Ca+ exchangers that are running in the normal forward mode.
Since Na+/Ca2+ exchanger in the forward mode is an important regulator of cell Na+/Ca2+ (Blaustein and Lederer, 1999), keeping it within physiological limits, it would not be prudent to block this mode of the exchanger. Therefore, a inhibitor of the Na+/Ca2+ exchanger that prevents the reverse but not forward mode of transport could spare ischemic/hypoxic retinal ganglion cells that are overloaded with Na+ while allowing relatively healthy cells (that posses exchangers running in the forward mode) to regulate intracellular Ca2+.
Recently a compound has been developed that selectively targets the reverse mode of the Na+/Ca2+ exchanger (Iwomoto et al, J. Biological Chem. 271:22391-22397 1996). This compound named KB-R743 ((2-[2-[4-(4-nitorbenzyloxy)phenyl]ethyl]isothiourea methanesulfonte) has been found to preferentially block the reverse mode of the Na+/Ca2+ exchanger in a variety of cell types including neurons (Breder et al, 2000, Iwomoto et al, 1996.). In addition, cyclosporin A as been identified as having made a specific activity in the inhibition of sodium-calcium exchange (Biophysical Journal UCI 80, No. 1, part 2 of 2, January 2001).
Although in glaucoma, the sequence of pathological events leading to the death of RGCs and the optic nerve tract; is not known, it is likely to involve a lethal increase in intracellular Ca2+. If, in the optic nerve, this increase in Ca2 + is the result of a reversal of Na+/Ca2+ exchanger, subsequent to depolarization and/or Na+ overload (mediated by non-inactivating Na channels) then it should be possible to spare axons and their RGCs by blocking reversal of the Na+/Ca2+ exchanger. Therefore blockade of the reverse mode of the Na+/Ca2+ exchanger may prevent or reduce the loss of optic nerve tract fibers associated with glaucoma.
Accordingly, a method in accordance with the present invention for preventing retinal ganglion cell death associated with glaucoma in an animal of the mammalian species, including humans, includes the steps of administering to the ganglion optic nerve of said manual, a pharmaceutical composition which comprises as its active ingredient, one or more compounds having activity for blocking a reverse mode of the sodium/calcium exchanger.
The invention provides a method for altering a plausible sequence for pathological events in retinal ganglion optic cells associate with glaucoma in order to prevent retinal ganglion cell death. The sequence includes the pathological depolarization of retinal ganglion cells, and influx of millimolar amounts of sodium via non-inactivating the sodium channels and a subsequent reversal of the sodium/calcium exchanger, mediated by both membrane depolarization and increased intracellular sodium causing a toxic buildup of intracellular calcium. Specifically, the method comprises the blocking of a reverse mode of the sodium/calcium exchanger in retinal ganglion cells in order to prevent buildup of the calcium level in the retinal ganglion cells to a lethal level.
In accordance with the present invention, the method is provided for maintaining normal intracellular calcium in ganglion cells following a period of anoxia. The method comprises contacting the ganglion neuronal cells with a composition for blocking of a reverse mode of the sodium/calcium exchanger in the retinal ganglion neuronal cells. Thus, the present invention provides a method for providing a neuroprotective effect to retinal ganglion cells in the eye of the human.
More specifically, the method includes the administration of a pharmaceutical composition comprising ((2-[2-[4-(4-nitorbenzyloxy)phenyl]ethyl]isothiourea methanesulfonte) to an eye. In addition, the method may include the pharmaceutical composition comprising of cyclosporian A and additional compositions and compounds which may be determined by a screen as hereinafter set forth.